1. Field of the Invention
This invention relates to a phosphor, and more particularly to a rare earth element activated complex halide phosphor.
2. Description of the Prior Art
Some kinds of phosphors emit fluorescent light when exposed to (excited by) visible light or infrared rays after exposure to ionizing radiations, ultraviolet rays, cathode rays, or the like. This kind of excitation is referred to as "stimulation", and the phosphor which emits fluorescent light upon stimulation is referred to as "stimulable phosphor". It is known that a stimulable phosphor can be used for a radiation image storage panel. That is, a radiation image of an object can be obtained by causing a panel composed of a stimulable phosphor to absorb a radiation passing through the object, exposing the panel to visible light or infrared rays to cause the stimulable phosphor to release the energy of the radiation stored therein as fluorescent light, and then detecting the fluorescent light.
In most cases of practical use of a radiation image storage panel, the object of the image is a human body. Accordingly, particularly when ionizing radiations such as X-rays or .gamma.-rays are used, it is required for a stimulable phosphor employed in the radiation image storage panel to emit light of as high luminance as possible upon stimulation thereof to decrease a dose of ionizing radiations to which the object is exposed.
As a stimulable phosphor, a rare earth element activated barium fluorohalide phosphor represented by the following formula has been known in the art (see U.S. patent application Ser. No. 57,091); EQU BaFX:yLn
wherein X is at least one halogen selected from the group consisting of chlorine, bromine and iodine, Ln is at least one rare earth element selected from the group consisting of europium, cerium and terbium, and y is a number satisfying the condition of 0.ltoreq.y.ltoreq.0.2.
Further, it has been known that a rare earth element activated barium fluorohalide phosphor represented by the following formula emits light of high luminance upon stimulation thereof; EQU BaF.sub.2.aBaX.sub.2 :zLn
wherein X and Ln have the same definitions as the above, and a and z are numbers satisfying the conditions of 0.90.ltoreq.a.ltoreq.1.05 and 0.ltoreq.z.ltoreq.0.4, respectively. When a and z are a=1 and 0.ltoreq.z.ltoreq.0.2, respectively, the phosphor corresponds to the above-mentioned rare earth element activated barium fluorohalide phosphor described in U.S. patent application Ser. No. 57,091.
Furthermore, it has been known that a rare earth element activated barium magnesium fluorohalide represented by the following formula emits light of high luminance upon stimulation thereof; EQU BaF.sub.2.aBaX.sub.2.bMgF.sub.2 :fLn
wherein X and Ln have the same definitions as the above, and a, b and f are numbers satisfying the conditions of 0.90.ltoreq.a.ltoreq.1.05, 0.ltoreq.b.ltoreq.1.2 and 10.sup.-6 .ltoreq.f.ltoreq.0.03, respectively.
As is clear from the above description, it has heretofore been known that a rare earth element activated divalent metal fluorohalide phosphor represented by the following formula emits light of high luminance upon stimulation thereof; EQU BaF.sub.2.aBaX.sub.2.bMgF.sub.2 :fLn
wherein X is at least one halogen selected from the group consisting of chlorine, bromine and iodine, Ln is at least one rare earth element selected from the group consisting of europium, cerium and terbium, and a, b and f are numbers satisfying the conditions of 0.90.ltoreq.a.ltoreq.1.05, 0.ltoreq.b.ltoreq.1.2 and 10.sup.-6 .ltoreq.f.ltoreq.0.03, respectively. When the above rare earth element activated divalent metal fluorohalide phosphor is employed in a radiation image storage panel, in view of the above-mentioned circumstances, the phosphor is desired to emit light of as high luminance as possible upon stimulation thereof.